In a synchronous digital data transmission system, the sequence of bits to be transmitted is first converted into a sequence of symbols. These symbols are then transmitted one at a time at instants called signaling instants, which have a T-second spacing and are determined by the transmitter clock. A carrier wave modulation technique is used wherein each symbol is caused to correspond to a discrete value of one or more characteristics (e.g., amplitude, phase) of the carrier wave. The modulated carrier wave is sent over the transmission channel. The modulated carrier is representative of the data at the signaling instants only, and it is essential, in order for the data to be correctly detected, that the receiver include an accurate clock indicating the signaling instants at which the signal received from the transmission channel is to be sampled. The phase and frequency of the receiver clock must be continuously adjusted, or synchronized, to optimize the sampling instants of the received data signal, and to compensate for phase and frequency variations between this clock and that of the transmitter. The synchronization of the receiver clock actually comprises three distinct operations.
A first synchronization operation which is performed before the first transmission of data takes place on a given day, for example, in the early morning. During this phase the transmitter provides a synchronization signal with which the receiver clock synchronizes. This operation may be relatively slow since it is performed only once a day.
A second transmitting synchronization operation which is carried out before each data message is transmitted. During this phase the receiver clock synchronizes with the synchronization signal provided by the transmitter. This operation must be very fast since the time required to achieve synchronization must be much shorter than that needed to transmit the actual data message if a satisfactory throughput is to be obtained.
A final operation is performed during the transmission of data for the purpose of maintaining synchronization. During this phase the receiver clock is continuously adjusted in accordance with a timing information derived from the received data signal.
This invention deals with the initial synchronization of the receiver clock. The invention is particularly well adapted to the requirements of the second synchronization operation defined above because it enables the various steps involved to be performed very quickly, but of course it can also be used for achieving the first synchronization operation.
In a data receiver, the pulses that define the sampling instants are provided by a clock pulse generator, or clock, the phase and frequency of which are adjusted by means of timing information supplied by a timing recovery device. Such devices may be regarded as falling within one of two main classes.
The first class includes those timing recovery devices in which timing information is obtained by filtering out a spectral line at the signaling frequency 1/T Hz or at some multiple of that frequency. This type of device is described, for example, in an article entitled, "Statistical Properties of Timing Jitter in a PAM Timing Recovery Scheme," by L. E. Franks and J. P. Bubrouski, in IEEE Transactions on Communications, Vol. COM-22, No. 7, July 1974, pp. 913-920. Briefly, in the timing recovery device described in said article, the signal received from the transmission channel, whether it is the synchronization signal supplied during any of the initial synchronization operations or the data signal being transmitted, is multiplied by itself and is then fed to a narrow-band filter centered at the signaling frequency. This filter provides a sine wave at the signaling frequency which is used as a timing wave with which the clock pulse generator synchronizes.
The timing recovery devices in this first class are very sensitive to noise. In addition, the narrow-band filters used in conjunction with digital techniques have a relatively long response time which is not conducive to achieving a fast initial synchronization. Accordingly, the use of said devices has been limited to repeaters and low-speed modems.
The second class includes those timing recovery devices in which the received signal is processed to obtain a control signal which is then used to adjust the phase and the frequency of a phase-locked oscillator acting as a clock pulse generator. Such a device is described, for example, in French Pat. No. 75 14020 filed by the present applicant Apr. 25, 1975 (publication No. 2,309,089). The device described in the patent includes first and second filters which are respectively used to extract from the received signal a first signal S1 of frequency f.sub.1 =f.sub.c -1/2T and of phase .phi..sub.1, and a second signal S2 of frequency f.sub.2 =f.sub.c +1/2T and of phase .phi..sub.2, where f.sub.c is the carrier frequency and 1/T is the signaling frequency, and means for combining these first and second signals to provide an error signal representative of the phase difference .phi..sub.2 -.phi..sub.1. The error signal is used to adjust the phase of a phase-locked oscillator. During an initial synchronization operation, signals S1 and S2 are extracted from the received synchronization signal and combined to obtain the value of the phase difference .phi..sub.2 -.phi..sub.1 which is used as an initial adjustment value of the phase of the phase-locked oscillator. The timing recovery device briefly described above enables the initial phase of the receiver clock to be fairly quickly obtained during an initial synchronization operation. For example, in the case of data transmitted at 4800 bits per second in accordance with CCITT Recommendation V27, the initial phase of the clock can be obtained within sixteen signaling periods, assuming that an unconditioned type of transmission channel is in use. In a multipoint data transmission system, it is essential that the initial synchronization operation be performed as quickly as possible; accordingly, this invention aims at providing means for achieving a still faster initial synchronization.